1. Field of the Invention
The present invention relates to an IP telephone system having a call holding function and a callback function. For example the present invention can be adapted to the IP telephone system which manages emergency calls, such as a call which reports occurrence of crime, accident, or fire disaster or a call which requests a help of emergency crews.
2. Description of Related Art
As a phone service, an emergency call service is known. An emergency call is a call which reports occurrence of crime, accident, or fire disaster or a call which requests a help of emergency crews. In order to deal with spread of mobile communication networks and IP telephone networks in resent years, a study of those networks to be adaptable to manage emergency calls is proceeded by the Japanese Ministry of Internal Affairs and Communications for example. As documents showing the study results of the Japanese Ministry of Internal Affairs and Communications, the following Documents 1 and 2 are known.
In the emergency call service, it is preferable that a call connection is remained until a receiver (that is, an emergency call center) disconnects the call connection even after a caller puts his or her telephone terminal in an on-hook state. This function is referred to as a hold function. The hold function is provided so that the emergency call center is able to call back the caller who puts the telephone terminal in an on-hook state certainly. A function for calling from the emergency call center to the caller in use of the holding call connection is referred to as a callback function. The hold function prevents the possibilities that the telephone network is congested or the line for callback is busy on another call from a third party. Accordingly, the callback function can certainly be implemented.
A method for realizing a hold function and a callback function of the emergency call service in use of the Public Switched Telephone Network is disclosed, for example, in the following Document 3. According to a method disclosed in the Document 3, a caller side telephone exchange determines whether the call is a normal call or an emergency call when receiving a calling signal. Then, when the call is an emergency call, the exchange keeps the call connection in a hold state without disconnecting even if the caller puts the telephone terminal in an on-hook state. Further, when the call connection is in the hold state, the exchange does not connect to the caller even when another call to the caller occurs.
Document 1: Draft report by the “Technical conditions related to the function of providing location information on callers sending emergency messages from cellular phones”, May 2004, Committee for the Advancement of Emergency Message System, Information and Communications Technology Sub-Council, Information and Communications Council, Ministry of Internal Affairs and Communications
Document 2: Report by the “Study Group on IP Network Technology”, February 2002, Study Group on IP Network Technology, Ministry of Internal Affairs and Communications
Document 3: Japanese Patent Laid-Open Publication No. 1999-103349
In the Public Switched Telephone Network, call connection is performed by physically allocating communication channels. Therefore, the hold function and the callback function can be realized with the method disclosed in Document 3. On the other hand, the IP telephone system performs communications of digital audio data in use of communication packets. Therefore, physically allocated communication channels are not used in the IP telephone system. Accordingly, in a conventional IP telephone system, a call connection cannot be on hold.
In the Public Switched Telephone Network, a telephone set is fixed so that the telephone set is managed in a single exchange at any time. On the other hand, in the IP telephone system, a telephone set is movable so that the exchange which manages the telephone set may be changed. When the exchange which manages the telephone set is changed, the receiver side exchange cannot recognize the caller side exchange. Therefore, in the conventional IP telephone system, when an exchange connected to a telephone set is switched, receiver cannot perform call back.